The present invention pertains to a dual-reflector antenna, particularly a microwave antenna normally used for mobile telecommunications networks.
In order to create a more compact system, one utilizes dual-reflector antennas, in particular those of the Cassegrain type. The dual reflector comprises a primary concave reflector, most commonly parabolic, and a secondary convex reflector, of much lesser diameter, placed in the vicinity of the focus of the parabola on the same axis of revolution as the primary reflector. A feed device comprising a waveguide is located along the antenna's axis of symmetry, facing the subreflector. These antennas are so-called “deep dish” antennas with a low F/D ratio, less than or equal to 0.25. In this report, F is the focal distance of the reflector (the distance between the reflector's apex and its focus) and D is the reflector's diameter.
These antennas exhibit high spillover losses and decrease the antenna's front-to-back ratio. Overflow losses lead to environmental pollution through RF waves and must be limited to levels defined by standards. One customary solution is attaching to the periphery of the primary reflector a shroud which has the shape of a cylinder, whose diameter is close to that of the primary reflector and of suitable height, coated on the inside with an RF radiation absorbing layer. The use of an expensive absorbent shroud is necessary to cancel out the spillover effect.
Furthermore, for low frequencies below 23 GHz, the high diameter of the primary reflector increases the levels of the secondary lobes (masking effect).